1. Field of the Invention
The present invention relates to a pneumatic radial tire which is characterized by a structure of a bead portion and has a high durability. The present invention is particularly useful for a pneumatic radial tire for a heavy load.
2. Description of the Related Art
Conventionally, in a pneumatic radial tire which is particularly used in a vehicle having a heavy weight, such as an industrial vehicle or a construction vehicle, there has been fear that a trouble such as separation is caused from a wound-up end of a carcass ply wound up around a bead core. Accordingly, it is generally carried out to intend an improvement of a durability of the bead portion by arranging a reinforcing layer called a chafer along the carcass ply. Patent Document 1 and 2 disclose such a chafer.
Further, in Patent Document 3 by the applicant of the present application, in order to suppress a shear strain of the wound-up end of the carcass ply so as to further improve the durability of the beadportion, there is proposed a bead structure in which a rubber filler 3b arranged in an outer side in a tire diametrical direction of a bead core 3a includes a lower filler 31 which is made of a hard rubber and has a round cross sectional shape, and an upper filler 32 which is softer than the lower filler 31, as shown in FIG. 6. A nylon chafer 35 including a nylon cord is arranged in an outer side of a carcass ply 34 which is wound up around the bead core 3a. 
In order to prevent separation beginning at a wound-up end 34E of the carcass ply 34, it is effective to employ a steel chafer including a steel cord (one example of a metal cord) in place of a nylon chafer 35. However, since a stress acting on the wound-up end is hard to be dispersed in the steel chafer in comparison with the nylon chafer, and there is fear that the separation is generated from the wound-up end, it is necessary to set the wound-up height low in the case of employing the steel chafer 36, as shown in FIG. 7.
It has been found that in the structure of FIG. 7, separation is easily generated in an inner position IA of the wound-up portion of the carcass ply 34, although it is possible to reduce a trouble in the wound-up end of each of the carcass ply 34 and the steel chafer 36. As a result of a repeated study of the inventor about a generation mechanism thereof, there has been found that a rigidity of an upper portion becomes lower than a lower portion on the boundary of a virtual line AL, in the bead structure as shown in FIG. 7, and the separation tends to be generated in the inner position IA due to a rigidity step in a tire diametrical direction.
Specifically, in the structure of FIG. 7, since a wound-up height of the chafer is lower in comparison with the structure of FIG. 6, the steel chafer 36 having a high strength is employed, the hard lower filler 31 is formed into the round cross sectional shape, and an interface with the upper filler 32 extends in a transverse direction, there is a tendency that the rigidity step in the tire diametrical direction becomes larger on the boundary of the virtual line AL. As a result, a deformation in the vicinity of the inner position IA becomes larger at the time of traveling, an adhesion property of the inner position IA is lowered by a rubber fatigue going with a long term use, and the separation is generated.
On the contrary, in the bead structure as illustrated in FIG. 2 of Patent Document 1 and FIG. 1 of Patent Document 2, the rigidity step in the tire diametrical direction does not appear very much even by arranging the steel chafer in the same manner. In these bead structures, the bead filler is divided into the lower portion and the upper portion, and the lower portion positioned just above the bead core is formed by the harder rubber than the upper portion, however, the hard rubber sharps so as to be tapered toward an outer side in the tire diametrical direction, and the rigidity of the bead portion changes slowly.